1. Field of the Invention
The present invention relates to a picture tube device including a field-emission cold cathode.
2. Description of Related Art
A field-emission cold cathode uses an electron-emitting material at room temperature unlike a hot cathode, which heats an electron-emitting material at a high temperature ranging from 750° C. to 1000° C. Therefore, a picture tube device including such a field-emission cold cathode does not have a problem of electron emission caused by barium evaporation, which is often problematic in the hot cathode.
FIG. 8 illustrates a conventional example of a picture tube device including a field-emission cold cathode (JP 9(1997)-204880 A). Numeral 31 denotes an electron gun, which includes a triode portion 32 formed of a field-emission cold cathode (also referred to as a field emitter array) 25, a first electrode 26 and a second electrode 27, and a main lens portion 28 for focusing an electron beam emitted from the field-emission cold cathode 25. The first electrode 26, the second electrode 27 and the main lens portion 28 have an aperture for allowing an electron beam to pass through.
FIG. 9 illustrates a configuration of the field-emission cold cathode 25. As shown in this figure, the field-emission cold cathode 25 includes a concave upper electrode 36, a plurality of electron-emitting electrodes 35 and a lower electrode 33 that is connected electrically to the electron-emitting electrodes 35. In the upper electrode 36, a sunken bottom portion of the concavity is provided with a plurality of apertures surrounding the electron-emitting electrodes 35 respectively, while a raised portion of the concavity surrounds the region where the plurality of apertures are formed (an emitter region). Numeral 34 denotes an insulating layer for electrically insulating the lower electrode 33 and the upper electrode 36 from each other. The upper electrode 36 is connected electrically to the first electrode 26 (see FIG. 8).
An electric field formed by the upper electrode 36 and the electron-emitting electrodes 35 forces the emission of electrons in the electron-emitting electrodes 35 as an electron beam, which forms a crossover 24 between the first electrode 26 and the second electrode 27 due to an electrostatic lens effect as shown in FIG. 10. Thereafter, the electron beam passes through the main lens portion 28, and forms a beam spot on a phosphor screen 18 (see FIG. 8).
In the field-emission cold cathode 25, by mounting the electron-emitting electrodes 35 more densely, it is possible to increase the beam current density, which is an electron emission amount per unit area of the cathode. Furthermore, it is to be expected that a technology will be developed for achieving a higher resolution of the picture tube device by utilizing the high beam current density characteristics and reducing a beam spot diameter.
The higher-density mounting of the electron-emitting electrodes 35 is realized by a microfabrication technique of a semiconductor process. With this technique, it is possible to increase the beam current up to at least about five to ten times as great as that in the picture tube using the conventional hot cathode.
However, when the beam current is increased, the current density at the crossover 24 increases and causes the electrons to repel one another by a space charge repulsion, leading to an increase in the beam spot diameter.
Moreover, when the beam current is changed for brightness modulation, for example, the crossover 24 is displaced, causing a so-called focus tracking.